African Swine Fever (ASF)

Occurrence: Sub Saharan African countries where there is a reservoir in warthogs, excluding South Africa, some Indian Ocean Islands, Georgia, Russia and Eastern European countries where it is mainly present in wild boar, also Sardinia.Age affected: All ages.Cause: African Swine Fever virus.Effects: Highly contagious and often fatal disease, beginning 4-5 days after infection and causing fever followed by dullness, breathing difficulty, vomiting, coughing, nasal and ocular discharge, abortion in pregnant sows, cyanosis of the extremities and death within 7 days. Chronically affected pigs are emaciated and often lame with skin ulceration.

Causes

African Swine Fever virus is a DNA virus belonging to the Asfivirus family. At least 22 types exist which enables outbreaks to be traced to source. It is extremely resistant to putrefaction and sunlight, and can persist in refrigerated meat and carcasses for up to 6 months and for much longer when frozen.

Mode of transmission

Infection is spread from pig to pig usually by aerosol from infected discharges and faeces, but also by the consumption of infected meat which has been inadequately cooked, by the bites of soft ticks, the bites of lice and flies and by direct inoculation from contaminated syringes. Infection can also be spread on contaminated implements and during transport. The virus can survive in carrier pigs, in infected buildings and for up to 4 years in infected Ornithodorus ticks.

A pig that is affected by ASF - one of the characteristics is the occurrence of petechias.Photo credit: Lina Mur.

Clinical signs

African Swine Fever is highly contagious and infection spreads rapidly through a unit, with clinical signs of fever beginning 4-5 days after infection and causing fever followed by dullness, breathing difficulty, vomiting, coughing, nasal and ocular discharge, abortion in pregnant sows, cyanosis of the extremities and death within 7 days. Chronically affected pigs are emaciated and often lame with skin ulceration. The high morbidity rate and accompanying high mortality are suggestive of one of the Swine Fevers.

Postmortem lesions

The carcass often has cyanosis of the extremities. Haemorrhages are widespread throughout the body and the lymph nodes may be so haemorrhagic that they resemble pieces of spleen. Haemorrhages on the surface of the heart may bleed into the pericardial fluid and those on the pleura bleed into the pleural cavity. Haemorrhages are frequently present on the lungs, heart, liver, kidney and bladder, the hepatic lymph node is haemorrhagic in all cases and the gall bladder is oedematous. In chronic cases there may be arthritis, pleuritic, pneumonia and skin ulcers. There are haemorrhages on the skin of aborted foetuses and on the placenta, if present. These findings suggest the presence of one of the Swine Fevers, but laboratory confirmation of the presence of the virus is required for diagnosis.

Post mortem performed on a pig that died of ASF in early 2012.Photo credit: Peter Evans.

Treatment and prevention

There is no treatment. Prevention is by means of a slaughter, carcass destruction and disinfection policy once the presence of the disease has been confirmed. The disease is Notifiable and the veterinary services of the country concerned are responsible for its control, prohibiting pig movement, supervising the slaughter of all animals on the affected farm, the destruction of their carcasses and the thorough cleaning and disinfection of the premises. All pigs in contact are traced and examined for signs of disease and meat from affected animals is destroyed. Food waste is currently destroyed in Europe, but, where it is fed to pigs, must be sterilised before feeding to pigs. Although vaccines have been described, and genetically-resistant pigs developed, neither are currently used for control.

Special note

African Swine Fever is one of the most important diseases of pigs and the identification of the disease in a country normally results in the suspension of exports of pigs and pig products. This disease poses no risk to human health.

Liver from ASF infected animal. The important pathological change is enlarged gallbladder which is pathogenomonic change for ASF.Photo credit: Iwona Markowska-Daniel.